1. Field
Aspects of the present disclosure relate to electrolytes for lithium secondary batteries and lithium secondary batteries including the same, and more particularly, to electrolytes that are used for lithium secondary batteries which, when used in a lithium secondary battery, improve high-temperature lifetime characteristics and high-temperature capacity conservation characteristics of the battery.
2. Description of the Related Art
Lithium secondary batteries are rechargeable, and the energy density per unit weight thereof is three or more times greater than that of conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, or nickel-zinc batteries. In addition, the charging rates of lithium secondary batteries are high. Due to such features, research and development of lithium secondary batteries are actively being performed.
Generally, a lithium secondary battery includes a cathode, an anode, and a separator and an electrolyte which are interposed between the cathode and the anode. Among these components, the electrolyte may further include a small amount of additive to improve performance of the lithium secondary battery, while retaining physical properties of the electrolyte. This additive addition method is drawing attention.
The additive has various functions. For example, the additive may form a solid electrolyte interface (SEI) for preventing direct contact between an electrode active material and the electrolyte. Additives for forming a SEI on the surface of an electrode can be classified as an anode additive for aiding formation of an SEI on the surface of graphite and an overcharge protection (OCP) additive for forming a film on the surface of a cathode.
The recent increasing demand for lithium secondary batteries having high energy density, for example, batteries for electric vehicles, has led to research into high voltage cathode active materials. However, research into an electrolyte additive for preventing oxidation of an electrolyte occurring at the surface of a cathode active material, has not yet been implemented.
In general, the potential window of an electrolyte needs to be wider than that between a cathode active material and an anode active material. However, in order to increase the energy density of a battery, an active material for high voltage has been used. Thus, the potential window of the electrolyte has become narrower than that between a cathode active material and an anode active material. Accordingly, decomposition of the electrolyte may be prevented by forming a film for preventing direct contact between the electrolyte and an electrode active material.
If a conventional aromatic compound, such as biphenyl or isomers of terphenyl are used as electrolyte additives, the electrolyte additive performs OCP by forming a thick film at the surface of the cathode when the voltage of the battery is equal to or higher than a reference voltage value so as to block passage of lithium ions and current flow. Recently, a method of forming a film at the surface of a cathode by using a low concentration of an additive has been introduced. However, the battery characteristics obtained were not satisfactory.